Good luck Sloth.
Frankly, I'm surprised your association is putting you in this position on a Friday night with no JH/JV experience.
I assume this is a varsity game, so your secretary is showing confidence in your ability, you need to be equally confident in yourself.
Best advice given so far...Slow Down. Rushing will lead to mistakes especially when your mechanics are green.
Hopefully you have a very good Umpire on your crew. If so, it'll make your job much easier.
Come in to where he is spotting the ball between downs to be near enough for a quick question to him or a comment from him before hustling back to your position. Ask him to remind you of the clock status before penalities and after 1st downs so you know whether to wind the clock on the ready.
Know what you're going to say to the coaches during pregame and to captains at the toss...write it down if you have to. It is during these meetings that you'll probably me the most nervous. Practice these conversations and the mechanics of indicating the results of the toss. Get through these meetings without making it obvious your a newbie....also, don't tell the coaches this is your first WH assignment.
Arrive a little earlier than normal to give yourself extra time and avoid being rushed.
Oh yeah! Bring a coin!
Good luck.